Mass spectrometer



Oct. 2, 1956 w. c. WILEY ET AL MASS SPECTROMETER 2 Sheets-Sheet 2 Filed Sept 8, 1952 ATTOR/VE Y Uit MASS SPECTROMETER William C. Wiley and Daniel B. Harrington, Detroit,

Mich., assignors to Bendix Aviation Corporation, Detroit, Mich., a corporation of Delaware Application September 8, 1952, Serial No. 308,441

13 Claims. (Cl. 250-413) This invention relates to mass spectrometers and more particularly to a mass spectrometer for providing strong output signals to indicate the masses of. the difierent ions in the spectrometer.

In many types of spectrometers, pulses of ions are utilized to determine the masses of the different gases or vapors in an unknown mixture and also to determine the relative abundance of the different gases or vapors in the mixture. In such spectrometers, a plurality of ions are formed from the different gases or vapors and are collected in a relatively confined region. After a relatively large number of ions have been collected, the ions are directed by an accelerating force towards a detector. This force produces a greater acceleration on the ions of relatively light mass than on the ions of heavy mass and causes the ions of light mass to reach the detector before the ions of heavy mass. By measuring the relative times at which the ions of different mass reach the detector, the masses of the different ions can be determined.

The strength of the signals produced at the detector by the ions of a particular mass is dependent upon the number of ions of that mass in the unknown mixture. Because of this relationship, the strength of the signals produced at the detector are sometimes relatively weak. Various systems have been proposed to increase the strength of the signals so as to provide the spectrometers with an increased sensitivity but these systems have not been entirely successful. For example, systems have been proposed in which an electron multiplier is used to detect the ions and to amplify the signals produced by the ions. However, such systerns have been costly and not entirely successful because the electron multiplier has had to be fairly large to receive all of the ions forming eachpulse.

This invention provides a time-of-flight mass spectrometer for providing sensitive indications of the masses of different ions in a pulse even when the number of'ions of a particular mass in the pulse is relatively small. The spectrometer provides such sensitive indications by utilizing a standard collector to receive the ions in the pulse and by utilizing the electrons emitted from the collector upon the impingement of ions to operate an electron multiplier. The electron multiplier then produces signals which indicate in considerably amplified form the relative times at which the ditferent ions reach it. The electron multiplier can be small and inexpensive because of the inclusion in the spectrometer of components for focussing the electrons as they travel towards the multiplier.

An object of this invention is to provide a mass spec-' trometer for producing pulses of ions and for measuring the time required for ions of different mass to travel through a predetermined distance. I

Another object is to provide a mass spectrometer .of the above character for measuring the masses of different ions with a sensitivity considerably increased over that obtained in spectrometers now in use.

A further object is to provide a mass spectrometer of the above character for utilizing the electrons secondarily 2 emitted from an ion collector to produce an amplification 1n the signals produced at. the collector.

Still another object is to provide a mass spectrometer ofthe above character for imposing predetermined magnetic and electrical fields on the electrons secondarily emitted from an ion collector so as to produce a focusing action on the electrons.

A still further object is to provide a spectrometer of the above character which is relatively simple, reliable and inexpensive.

Other objects and advantages will be apparent from a detailed description of the invention and from 'the append ed drawings and claims.

In the drawings Figure 1 is a somewhat schematic view, partly inbloclc -form and partly in perspective, illustrating one embodiment of the invention;

Figure 2 is a perspective view illustrating certain components shown in Figure 1 in relationship to other components forming a part of the invention; and

Figure 3 is a schematic view illustrating the operation of certain of thecomponents shown in Figures 1 and 2.

In one embodiment of the invention, a Wedge-shaped filament 10 made from a suitable material such as tungsten is provided. An electrode 12 is disposed relatively close to the filament Hand is provided with a vertical slot 14, the median position of which is at substantially the same horizontal level as the filament 10. An electrode 16 having a slot 18 corresponding substantially in shape and posily in front of the imaginary line disclosed above and in parallel relationship with the backing plate 22. The elec' trode 24 may be separated from the plate 22 by approximately two millimeters.

Top and bottom slats 28 extend between the plate .22 and the electrode 24 to form a compartment with the electrode 16 and the collector 20. A horizontal slot 30. is provided in the bottom slat 28. A conduit 32 communicates at one end with the slot 30 and at the other end with a receptacle 34 adapted to hold the different gases consti tuting an unknown mixture.

An electrode 36 is disposed at a relatively small distance," such as two millimeters; from the electrode 24 and in suh stantially parallel relationship to the electrode 24. g The electrode 36 is provided with a slot 38 corresponding substantially in shape and position to the slot 26. An electrode 40 made from a suitable wire mesh is disposed ata relatively great distance, such as forty centimeters, from the electrode 36 and is substantially parallel to the electrode 36; A collector 42 is disposed in substantially parallel relationship to'the electrode 40 and at a relatively small distance, such as two millimeters, from the electrodei' A permanent magnet having a pair of oppositely disposed pole pieces 48 and 50 (Figure 2) is adapted to produce a magnetic field of substantially uniform intensity between thepole pieces. The magnet is so positioned horizontally as to produce its magnetic field primarily only in the region between an electron multiplier-1' 52 and the electrode 40 and not in most of the region traversed by the ions in their movement from the elec-' of the pole'piece 48 lies in a plane above the plane- Patented Oct, 2, 195.6

defined by the upper extremities of the slots 26 and 38 and the face of the pole piece 50 lies in a plane below the plane defined by the lower extremities of the slots.

The electron multiplier 52 is positioned to one side of the electrodes 36 and 40 and at an intermediate position between the electrodes. The electron multiplier 52 is disposed in substantially perpendicular relationship to the electrodes 36 and 40 at substantially the same vertical level as the electrodes. The electron multiplier may be similar to that described on pages 831 and 832 of Radio Engineering (3rd edition, 1947) by Professor Frederick E. Terman. A time indicator 54, such as an oscilloscope, is connected to the electron multiplier 52 to indicate the times at which signals are produced by the multiplier.

The electrode 12 normally has a positive voltage ap plied to it through a resistance 56 from a suitable power supply 58. The collector also has a slightly positive voltage applied to it from the power supply 53 through a resistance 60. The collector 20 is biased at a slightly positive voltage to attract electrons flowing through the slot 18 from the filament 10. A negative voltage is applied to the collector 42 through a suitable resistance 62 from the power supply 58. The filament 10, the backing plate 22 and the electrode 24 are grounded through suitable resistances 64, 66 and 68, respectively, and the electrodes 16, 36 and are directly grounded.

In the steady state condition, the electrons emitted from the filament 10 are attracted towards the electrode 12 because of the positive voltage on the electrode relative to the voltage on the filament. The electrons are not further accelerated past the electrode 12 Since the electrode 16 is at a lower potential than the electrode 12. Because of this voltage relationship, relatively few, if any, electrons reach the region between the backing plate 22 and the electrode 24, and these electrons do not have sufficient energy to ioniz-e molecules of gas introduced into the region. a

When negative pulses of voltage are applied to th filament 10 and the electrode 12 through coupling capacitances 72 and 74, respectively from a pulse forming circuit 76, the voltage on the electrode 12 becomes lower than the voltage on the electrode 16. This causes the electrons passing through the slot 14 to be further acce'lerated in the region between the electrodes 12 and .1 16. Because of this additional acceleration, the electrons travel into the region between the backing plate 22 and the electrode 24 with sufficient energy to ionize molecules of gas introduced into the region from the receptacle 34.

The ions formed from the gas molecules are retained in the electron stream because of the opposite charge which they have relative to that provided by the electron stream. Since the electron stream has a relatively great charge, a considerable number of ions can be retained before the stream becomes saturated. The ions are retained in a relatively narrow-space because of the collimating action which is provided on the electron stream by the slots 14 and 18 and which maybe provided by a magnetic field (not shown). The action of the electron stream in retaining the ions within the stream is fully disclosed in co-pending application Serial No. 221,554, filed April 18, 1951, by Ian H. McLaren and William C. Wiley, now Patent No. 2,732,500.

The pulse forming circuit 76 is set to apply the pulses to the filament 10 and the electrode 12 for a particular period of time to allow a considerable number of ions to be produced for retention in the stream. When the pulses are cut off, the ions are available for easy withdrawal upon the application of voltage pulses to the electrodes 22 and 24 from the pulse forming circuit 76; The pulse forming circuit 76 is set to apply the pulses to the electrodes 22 and 24 at approximately the same instant that the pulses applied to the filament 10 and the electrode 12 are cut off. These voltage pulses cause the ions to be withdrawn in a pulse from their place of retention.

The pulses of voltage on the plate 22 and the electrode 24 produce an electric field of moderate intensity between the plate 22 and the electrode 24 and an electric field of considerable intensity between the electrodes 24 and 36. For example, a pulse of approximately +200 volts may be applied to the plate 22 and a pulse of approximately +15O volts may be applied to the electrode 24 while the electrode 36 is maintained at ground.

The imposition of the particular voltage pulses on the plate 22 and the electrode 24 provides a compensation for differences in the positioning and in the random motion of individual ions during their retention in the electron stream. The random motion of individual ions is produced by the thermal and other energies in the ions. These energies are instrumental in causing some of the ions to be moving towards the collector 42 and other ions to be moving away from the collector 42 at the time that the ions are withdrawn in a pulse from their place of retention. The compensation provided by the imposition of the particular voltage pulses on the plate 22 and the electrode 24 is fully disclosed in co-pendingapplication Serial No. 249,318, filed October 2, 1951, by William C. Wiley, now Patent No. 2,685,035.

The acceleration provided on the ions by the voltage pulses on the plate 22 and the electrode 24 is greater for ions of relatively light mass than for ions of heavy mass. Because of the different accelerations imparted to the ions, the ions of light mass travel faster than the ions of heavy mass and reach the collector 42 before the ions of heavy mass. As a result, the masses of the different ions can be determined by the relative times at which the ions reach the collector 42.

Because of the acceleration imparted to the ions in the region between the backing plate 22 and the electrode 36, the ions have considerable energy as they move past the electrode. The ions retain this energy during their movement between the electrodes 36 and 40 since the ground potential on both electrodes causes the region between them to have substantially no electric field. The energy of the ions increases still more as the ions move in the region between the electrode 4% and the collector 42 because of the negative voltage on the collector.

The considerable energy of the ions causes them to strike the collector with suflicient force to produce a secondary emission of electrons from the collector. The number of electrons secondarily emitted from the collector by the ions of a particular mass is substantially proportional to the number of ions of that mass which reach the collector. The electrons secondarily emitted from the collector 42 are withdrawn from the collector by an electric field provided between the collector and the electrode 40. This field may be either continuous or in pulse form and is produced by the application of a negative voltage of approximately volts on the collector 42 while maintaining the electrode 40 at ground. The continuous field may be provided by connecting the collector 42 to the power supply 58 through the resistance 62.

Most of the electrons secondarily emitted from the collector 42 are withdrawn from the collector in a direction substantially perpendicular to the magnetic field produced on the electrons by the pole pieces 48 and 50. Because of the perpendicular relationship between the direction of electron movement and the direction of the magnetic field, the magnetic field applies a force substantially perpendicular to it and the direction of electron movement. This force causes the electrons to move in arotary path, .as illustrated at 84, 86 and 88 in Figure 3 for individual electrons.

As illustrated in Figure 3, the electrons which move in an essentially perpendicular direction past the elec-- trode 40 become focused in the plane of the figure. This focusing action occurs because all of the electrons travel in circular paths having substantially constant and equal radii. For example, electrons travel in Figure 3 through the circular paths 84, 86 and 88 having constant and equal radii 90, 92 and 94, respectively. The focusing produced on the electrons causes the distance between them at the point of focus .to be considerably less than the distance between them at the collector 42.

The electron multiplier 52 is disposed substantially at the position of electron focus to receive the electrons traveling from the collector 42. The multiplier 52 receives difierent groups of electrons at relative times corresponding to the times at which ions of different mass impinge on the collector 42. The electron multiplier 52 considerably amplifies the signals produced by the difiierent groups of electrons. The amplification provided by the multiplier 52 may be as much as one million times the signals which would be produced by the electrons themselves.

In addition to the focusing action provided on the electrons and the subsequent amplification of the signals produced by the electrons, the apparatus disclosed above has several other important advantages. One advantage, of course, is that the electron multiplier 52 can be fairly small. Since the cost of an electron multiplier increases considerably with increases in size, the small size of the multiplier is an important factor.

Another advantage is that the electrons are removed by the magnetic field from the vicinity of the ions be-- In this fore they are collected by the multiplier 52. way, only electrons, and no ions, are collected by the multiplier. Since the multiplier 52 receives only electrons, it cannot be contaminated during operation.

It should be appreciated that the focusing action disclosed above can also be obtained by maintaining the collector 42 at substantially ground potential and by applying a positive voltage to the electrode 40. In this way, electrons emitted from the collector 42 are attracted towards the electrode 40 instead of being repelled from the collector. The electrons are then focused by the magnetic field provided between the pole pieces 48 and 50.

It should be further appreciated that ions of a particular mass can be discriminated from ions of all other mass by applying a pulse of voltage, instead of a direct voltage, to the collector 42. If the pulse of voltage is applied to the collector 42 at approximately the instant that the ions of the particular mass reach the collector 42, an electric field will be produced between the electrode 4!) and the collector 42 to withdraw only theclectrons produced by the ions of the particular mass. In this way, the spectrometer disclosed above can be very easily adapted for use as a leak detector.

Although the pulse forming circuit 76 is shown in block form in the drawings, it can be purchased or it can be easily built by a person skilled in the art. For example, Model 902 of the double-pulse generator manufactured by the Berkeley Scientific Company of Richmond, California, may be used to produce a plurality of pulses having the time spacings disclosed above. This model generator is fully disclosed in a publication entitled Instruction Manual, Berkeley Double Pulse Generator, Model 902 issued by the Berkeley Scientific Company in August 1950. Or equipment for producing pulses having such time spacings may be built in accordance with the principles outlined on pages 223 to 238, inclusive, of volume 20 entitled Electronic Time Measurements of the Radiation Laboratory Series prepared by the Massachusetts Institute of Technology. Furthermore, the circuit disclosed in co-pending application Serial No. 288,104, filed May 16, 1952 by William C. Wiley and Macon H. Miller may also be easily adapted for use in the spectrometer disclosed above.

Although this invention has been disclosed and illusciples involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

l. A mass spectrometer, including, means for producing a plurality of ions and storing them in a relatively confined region, means for imposing an accelerating force on the ions to withdraw the ions from their place of storage and to produce a separation of the ions on the basis of their mass, means for collecting the ions after their travel through a relatively great distance and for producing electrons upon the collection of the ions, means for withdrawing the electrons from the collecting means, means for focusing the electrons after they have been withdrawn from the collecting means, means for detecting the electrons and for amplifying the signals produced by the electrons, and means for indicating the relative times at which the electrons produced by the diiferent their withdrawal from the collecting means, an electron multiplier for receiving the electrons after their focusing and for producing amplified signals indicative of the electron reception, and means for indicating the relative times at which the electron multiplier produces signals so as to provide a determination of the masses of the ions and their relative abundance.

3. A'mass spectrometer, including, means for storing a plurality of ions, means for imposing an electric field on the ions to withdraw the ions in a pulse from their place of storage and to produce a separation of the ions.

on the basis of their mass, means for collecting the ions after their travel through a relatively great distance and for secondarily emitting electrons upon the impingement of the ions, means for imposing an electric field on the electrons to withdraw them from the collecting means, means for imposing a magnetic field on the electrons to produce a focusing action on them, an electron multiplier for receiving the electrons after they have been focused and for amplifying the signalsjproduced by the electrons,v and an indicator for showing the relative times at which different signals are produced by the electron multiplier so as to provide a determination of the masses of the different ions and their relative abundance.

4. A mass spectrometer, including, means for producing a plurality of ions and for storing them in a relatively confined region, means for withdrawing the ions in a pulse from their place of storage and for accelerating the ions to separate them on the basis of their mass, means for collecting the ions after their travel through a relatively great distance and for secondarily emitting electrons upon the collection of ions, means for accelerating the electrons from the collecting means, means for subjecting the electrons to a rotary movement to remove them from the vicinity of the ions and to provide a focusing action on them, an electron multiplier for detecting the electrons after their rotary movement and for amplifying the signals produced by the electrons,and means for indicating the relative times at which electrons produced by the different ions are detected.

5. A mass spectrometer, including, means for storing 1 a plurality of ions, means for subjecting the ions to an electric field to withdraw the ions and to produce a separation of the ions on the basis of their mass, means for collecting the ions after their travel through a relatively great distance and for secondarily emitting electrons upon the. collection of the ions, means for subjecting the electrons to an electric field substantially perpendicular to the collecting means to with-draw the electrons from the collecting means, means for subjecting the electrons to a magnetic field substantially perpendicular to the direction of electron travel to produce a rotary movement of the electrons, an electron multiplier for detecting the electrons after their movement through a predetermined angle and for amplifying the signals produced by the electrons, and means for indicating the relative times at which electrons produced by the different ions are detected.

6. A mass spectrometer, including, means for storing a plurality of ions, means for withdrawing the ions from their place of storage and for accelerating the ions so as to separate the ions on the basis of their mass, a collector for receiving the ions after their travel through a relatively great distance and for secondarily emitting electrons upon the reception of the ions, :1 grid spaced at a relatively small distance from the collector, means for applying an electric field in the region between the grid and collector to withdraw the electrons secondarily emitted from the collector, a magnet having a pair of pole pieces positioned to apply a magnetic field on the electrons substantially perpendicular to the direction of the electric field so as to produce a rotary movement of the electrons, an electron multiplier for receiving the electrons after their travel through a predetermined angular distance and for amplifying the signal produced by the electrons, and means for indicating the relative times at which the electrons produced by the different ions are received by the multiplier.

7. A mass spectrometer, including, a backing plate, a first grid disposed at a relatively close distance to the backing plate, means for producing an electric field between the backing plate and the first grid to withdraw the ions from their place of storage and to produce a separation between them on the basis of their mass, a collector disposed at a relatively great distance from the first grid to receive the ions and to secondarily emit electrons upon the reception of ions, a second grid disposed between the first grid and the collector at a relatively close distance to the collector, means for producing an electric field between the second grid and the collector to withdraw the electrons from the collector, a magnet having a pair of pole pieces positioned to produce a magnetic field substantially perpendicular to the direction of electron travel so as to subject the electrons to a rotary movement, an electron multiplier for receiving the electrons after their rotary movement and for amplifying the signals produced by the electrons, and an indicator for showing the relative times at which the electrons produced by the difierent ions are received.

8. A mass spectrometer, including, a backing plate, a firstelcctrode disposed at a particular distance from the plate, means for providing a plurality of ions in the region between the backing plate and the first electrode, means for imposing an electrical field on the ions in the region between the backing plate and the first electrode to produce a movement of the ions from the region in a first direction substantially perpendicular to these members and to produce a separation of the ions on the basis of their mass, a collector disposed at a particular distance from the first electrode to receive the ions and to produce a substantially proportionate number of electrons, a second electrode disposed between the first electrode and the collector, means for imposing an electrical field on the electrons in the region between the second electrode and the collector to produce a movement of the electrons fromthe collector in a direction substantially opposite to the movement of the ions, magnetic means disposed to produce a magnetic field in a second direction having a component substantially perpendicular to the first direction for a focussing action on the electrons,

a detector disposedin a third direction having a'component substantially perpendicular to the first and second electrodes to produce amplified signals from the electrons, and means for indicating the relative times at which the electrons produced from the ions of difierent mass are detected.

9. A mass spectrometer, including, means for providing a plurality of ions, an electrical circuit for imposing an electrical field upon the ions to withdraw the ions from their place of provision and to produce a separation of the ions on the basis of their mass, a collector for receiving the ions after their travel through a particular distance and for producing a number of electrons substantially proportionate to the number of ions which are received, an electrical circuit for imposing an electrical field, upon the electrons to withdraw them from the collector, magnetic means for focussing the electrons after their movement from the collector, electrical means for detecting the electrons and for amplifying the signals produced by the electrons, and means for indicating the relative times at which the electrons produced by the difierent ions are detected.

10. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the plate, means for providing a plurality of ions in the region between the backing plate and the first electrode, an elec trical circuit for producing an electrical field between the backing plate and the first electrode to produce a movement of the ions from their place of provision and a separation of the ions on the basis of their mass, a collector disposed at a particular distance from the first electrode to receive the ions and to produce a substantially proportionate number of electrons, a second electrode disposed between the first electrode and the collector, an electrical circuit for producing an electrical field between the second electrode and the collector to withdraw the electrons from the collector, magnetic means for producing a magnetic field in a. direction having a component substantially perpendicular to the movement of the electrons to produce a curvilinear movement of the electrons and a focussing of the electrons, an electron multiplier disposed at substantially the position of optimum focus of the electrons, and means for indicating the relative times at which the electrons produced from the ions of different mass are detected.

11. A mass spectrometer, including, a backing plate, an electrode disposed at a particular distance from the plate, means for providing a plurality of ions in the region between the backing plate and the first electrode, an electrical circuit for imposing an electrical field upon the ions to withdraw them from their place of provision and to impart to the ions velocities dependent upon their mass, a collector disposed at a particular distance from the electrode to receive the ions and to emit a number of electrons substantially proportionate to the number of ions which are received, a magnet having a pair of pole pieces disposed to impose a magnetic field on the electrons in a direction having a component substantially perpendicular to the movement of the electrons from the collector, electrical means for operating on the electrons in conjunction with the magnetic field to produce a curvilinear movement of the electrons and a focussing action on the electrons, an electron multiplier disposed at substantially the position of optimum focus of the electrons to receive the electrons and to produce amplified signals, and means for indicating the relative times at which the ions of different mass are detected.

12. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the plate, means for providing a plurality of ions in the region between the backing plate and the first electrode, means for imposing an electrical force on the ions to produce a movement of the ions from their place of provision and a separation of the ions on the basis of their mass, a collector disposed at a particular distance from the first electrode to receive the ions and to produce a plurality of electrons substantially proportionate to the number of ions which are received, means for applying an electrical force on the electrons to withdraw the electrons from the collector, means for applying a magnetic force on the electrons in a direction having a component substantially perpendicular to the direction of movement of the electrons from the collector to produce an angular movement of the electrons into a position of optimum focus, an electron multiplier for detecting the electrons upon their movement to the position of optimum focus and for amplifying the signals produced by the electrons, and means for indicating the relative times at which the electrons produced from the ions of different mass are detected.

13. A mass spectrometer, including, means for provid ing a plurality of ions, means for imparting to the ions energies dependent upon the masses of the ions to produce a separation of the ions on the basis of their mass, a collector disposed at a particular distance from the place of ion provision to receive the ions and to emit a substantially proportionate number of electrons, an electrode disposed at a particular distance from the collector on the side of the collector facing the emission of electrons, an

electrical circuit for imposing an electrical field on the electrons in the region between the electrode and the collector to produce a movement of the electrons from the collector, a magnet having a pair of pole pieces disposed to produce a magnetic field in a direction having a component substantially perpendicular to the direction of movement of the electrons for a movement of the electrons in a rotary path and in a focussing relationship, an electron multiplier disposed at substantially the position of optimum focus of the electrons to produce amplified signals from the electrons, and means for indicating the relative times at which the electrons produced from the ions of different mass reach the electron multiplier.

References Cited in the file of this patent UNITED STATES PATENTS Berry Oct. 5, 1954 

